1.Introduction

Environmental changes in urban areas include reduced evapotranspiration
of precipitation and the transformation of up to 95% of net incident radiation
to heat (compare fig. 1 global radiation balance and fig. 2 urban radiation).
As a result, air temperatures inside buildings also rise and lead to discomfort or increased energy consumption for climate
management. A logical solution to create more comfortable air temperatures
inside and outside of buildings is to green their facades and roofs, thereby
„consuming“ this energy by evapotranspiration.

Fig. 2:Radiation balance of a black asphalt roof
as an example for urban radiation changes

Worldwide, the reduction of energy consumption for cooling and
ventilation installations is becoming increasingly important. While political
agreements announced the reduction of CO2 emissions for Germany by 30% until
the year 2020, CO2 emissions due to cooling are estimated to increase by 250%,
mainly because of the increased use of conventional air conditioning systems
(Fig. 3).

Fig. 3:Increase of CO2 emissions due
to cooling in Europe (after EECCAC 2003)

A new European
parliament directive on the energy performance of buildings (2002/91/EC),
strongly encourages the implementation of passive cooling techniques to improve
indoor climatic conditions as well as the immediate microclimate.

The Institute of
Physics, a project of the Architects Augustin and Frank (Berlin), is a research and
office building featuring several measures of sustainable architecture,
incorporating elements combining decentralized water management and reduced
energy consumption for cooling and ventilation. Rainwater is stored in cisterns
and used to irrigate a façade greening system and ventilation units using
evaporative cooling.Extra water is
collected in a pond in the building’s courtyard which allows the water to
either evaporate or drain into the ground.

2.Methodology

At the Technical
University of Berlin, section of Applied Hydrology and section of Building
Technology and Design, evaporation measurements are combined with radiation
measurements. Focus is he hydrologic change in urban areas compared to
naturalistic landscapes. Lysimeter measurements of the real evapotranspiration
of a meadow are completed through measurements on semi permeable surfaces,
greened roofs and green wall systems. Evapotranspiration measurements are
placed into relation to longwave and shortwave radiation measurements (Schmidt
2005).

Evapotranspiration
of a cubicmeter of water consumes about 680 kWh. This energy is transported as
latent heat into the atmosphere and released as longwave radiation when the
water vapour condenses as clouds. Parts of the longwave radiation is lost to
space, an important global energy loss for the received shortwave radiation
from the sun. A reduction of evapotranspiration in urban areas results in
higher surface temperatures and higher longwave emission. Second effect is a
higher sensible heat flux. The relation between sensible and latent heat flux
is expressed as the bowen ratio. The bowen ratio increases by reduced
evaporation rates.

The project in Berlin-Adlershof includes an ongoing monitoring of the
water consumption of different plant species of the façade greening system and
of resulting evaporative cooling and the effects on the energy balance of the
building. Temperature and radiation measurements assist in identifying the
economic and ecological benefits of the investigated climbing plants and
ventilation units.

3.Results

3.1Rainwater harvesting and stormwater management

Rainwater
harvesting becomes an important issue. Until 2020 another 1 billion people will
live in cities (UN/ DESA 2003). The increase of urban areas forces to give high
priority to decentralized measures of rainwater retention and new strategies in
water supply and wastewater management. Although in the natural landscape most
precipitation is evaporated or transpired, in urban areas, evapotranspiration
is greatly decreased and rainwater is instead swiftly directed into the sewer
system and to receiving waterbodies. The Institute of Physics Building is not
connected to rainwater sewers, as one main goal of the decentralized rainwater
harvesting is a retention of rainwater in order to reduce stormwater flows into
sewer systems. Rainwater is stored and used for the irrigation of the facade
greening system and adiabatic cooling systems. Storm water events with heavy
rainfall are managed with an overflow to a small constructed pond in one of the
courtyards, from which the water can evaporate or drain into the ground. To
protect the ground water quality, this drainage is only allowed through surface
areas with vegetation. Some of the roof surfaces are also extensively greened to
assist in retaining and treating water.

Fig. 4 and 5:façade
greening system (left), adiabatic exhaust air cooling in air conditioning
systems (right)

3.2Green Facades

Impermeable surfaces
like roofs and streets influence urban microclimates through radiation changes.
As a result of these changes, air temperatures inside buildings also rise and
lead to discomfort or increased energy consumption associated with climate
management. A logical solution to create more comfortable air temperatures
inside and outside of buildings is to green façades and roofs, thereby
„consuming“ this energy by evapotranspiration. According to measurements taken
at the UFA Fabrik in Berlin, extensive green roofs transfer 58% of net incident
radiationinto evapotranspiration
during the summer months. The annual average energy consumption is 81%, the
resultant cooling-rates are 302 kWh/(m²*a) with a radiation balance of 372
kWh/(m²*a) (Schmidt 2005).

A more demanding
solution is a façade greening system which has a higher direct effect on the
energy performance of a building than a greened roof. Green façades were
implemented at the Institute with two objectives: 1) to passively climatize the
building through shading and solar radiation and 2) to harness evapotranspiration
to improve the microclimate inside and around the building. Plants provide
shade during summer, while during the winter, when the plants lose their foliage,
the sun’s radiation is able to pass through the glass-front of the building. In
the summer month July until September 2005 the water consumption for the well
developed Wisteria sinensis increased
up to 420 liter per day for 56 planter boxes. This represents a cooling value
of 280 kWh per day for one of the courtyards.

Figure 6:
mean evapotranspiration of the façade
greening system in mm/day and correspondent cooling rates

The mean
evapotranspiration between July and August 2005 for the south face of the
building was between 5.4 and 11.3 millimeters per day, depending on which
floorthe planters were located (Figure
5). This rate of evapotranspiration represents a mean cooling value of 157 kWh
per day.

In selecting the
climbing plants an emphasis was placed on types that can grow in the extreme
conditions of planter boxes. Of the various plants tested the Wisteria sinensis
has proven to do the best. In addition a special system of irrigation and
different substrates have also been applied and studied. A factor in the
selection was an adequate capillary rise of the water through the irrigation
and substrate systems. Another aspect studied was the providing of a layer of
insulation to some of the planter boxes, to compensate for large shifts in
temperature and especially to help protect against very low winter temperatures.
This comparison revealed that insulation can lead to significant differences in
plant growth.

3.3Evaporative exhaust air cooling

Air conditioning in the Institute of Physics is achieved through seven
adiabatic climatization units. These units use rainwater to cool air through
the process of evaporation. This is a two step process. First, the rainwater is
evaporated to reduce the temperatures of the air leaving the building. In a
second step, fresh air entering the building is cooled as it passes across a
heat exchanger with cooled air on its way out. This process is sufficient to
maintain indoor temperatures of 21-22 °C with outside temperatures of up to 30
°C. When outside temperature exceed 30°C, indoor temperatures are maintained by
a conventional cooling system.

4.Conclusions

Reduced evaporation is the main hydrological difference between urban
and rural areas. Missing evapotranspiration increases the thermal radiation
caused by higher surface temperatures and increases the sensible heat. New innovative
rainwater projects focus on the necessity of evapotranspiration rather than
infiltration.

This research is designed to generate recommendations for an optimal and
economical management of the building’s mechanical systems, emphasizing an
innovative and sustainable relationship with the resources water and energy as
well as the reduction of operating costs. The evaluation, optimization and
documentation of the project results is expected to provide a basis for the
long-term implementation and further development of innovative and economic
technologies. Practical findings are incorporated into simulations designed to
understand the transferability of these techniques to different climatic
conditions and to determine applications for future projects, concerning the
fields of design, construction, operation and maintenance.

The urban and
global climate change is mainly resulted due to reductions in
evapotranspiration. The increase in urbanization continues at a rate of 1 km²
daily in Germany. The global daily loss of forests are 350 km² (GTZ 2007). This
represents 1.3 % of annual loss of the remained 30% of forests worldwide. The
latent heat flux and it’s importance on the temperatures seems to be absolutely
underestimated in all climate models.

5.References and Links

EECCAC (2003) Energy
Efficiency and Certification of Central Air Conditioners. REPORT for the DGTREN
of the Commission of the E.U, 2001, Volume 1, 52 pp.